1. Field of the Invention
The present invention relates to a method of drawing a pattern for magnetic transfer by rotating a substrate to be drawn to scan an electron beam in the circumferential direction. The method is favorable in particular for drawing an original pattern of a master disk for magnetic transfer to magnetic recording media to be mounted on hard disk drives (HDDs).
2. Description of the Related Art
Common HDDs conduct read-write of data using a magnetic head flying over a magnetic recording medium (a hard disk) at a height of about 10 nm. The bit information on the magnetic recording medium is stored in concentrically arranged data tracks. The magnetic head is positioned on the data tracks in the data read-write processes. Servo information for positioning the magnetic head is stored on the magnetic recording medium. The servo information is recorded concentrically to the data tracks with a constant angular spacing.
The servo information is generally written by means of a device called a servo writer that records servo signals while positioning the magnetic head by controlling an actuator arm using a push pin.
However, the method of writing the servo information by means of a servo writer has a problem of inaccurate positioning of the servo signals and a problem of increased cost due to fast obsolescence of the servo writer.
In order to cope with these problems, an off-line servo recording technique and a self-servo recording technique have been developed.
The off-line servo recording technique records servo signals using an off-line servo recording device before installing a magnetic recording medium into an HDD. This technique improves accuracy as compared with a servo writing method using a push pin.
The self servo recording technique preliminarily installs a magnetic recording medium that has reference servo signals written therein into an HDD, and then a magnetic head is positioned referring to the reference servo signals in the HDD, thereby final servo signals are written to a plurality of magnetic recording media. Recording quality in this technique is determined depending on the reference servo signals. This technique is effective in a device cost.
The reference servo signals employed in the self servo recording technique include a reference servo signal in a spiral mode as disclosed in U.S. Pat. No. 5,668,679, Japanese Unexamined Patent Application Publication No. 2006-147139, Japanese Unexamined Patent Application Publication No. 2001-243733, for example.
The recording technique of the reference servo signal in the spiral mode will be described with reference to FIG. 11. FIG. 11 illustrates a magnetic recording medium (a hard disk) 200 mounted on a spindle motor (not illustrated), a pivot-rotating actuator arm 13, a magnetic head 12, two crush stops 17 and 18, and a voice coil motor 14.
The actuator arm 13 is driven by exciting the voice coil motor 14, thereby moving the magnetic head 12 in a radial direction. While the magnetic recording medium 200 is rotated and the magnetic head 12 is moved from the outer circumference to the inner circumference of the magnetic recording medium 200 in a constant speed, a reference servo signal is recorded in a spiral track 100, as shown in FIG. 11. The spiral track 100 having the recorded reference servo signal includes embedded timing information (for example, by missing bit). Thus, a specified number of spiral tracks 100 are recorded on the magnetic recording medium 200.
The positioning process using the spiral reference signal is performed on the following basis. FIG. 12 illustrates an output waveform when the magnetic head 12 crosses over one spiral track 100 in a process of signal read-back on the same single circle on the magnetic recording medium 200 having spiral tracks 100 written thereon while floating the magnetic head 12.
As shown in FIG. 12, the read-back waveform has an overall configuration of a leaf and contains periodic timing signals 101. With radial movement of the magnetic head 12, the timing signal 101 does not shift, and the peak position 102 of the read-back signal shifts. An angular position of the magnetic recording medium 200 is detected by the timing signal 101, and an amount of movement in the radial direction of the magnetic head 12 can be known by the shift of the peak position 102 of the read-back waveform. The position information is used for positioning the magnetic head 12.
When a reference servo signal is recorded by an off-line servo writing process, for example, based on the above-described method and the magnetic recording medium is installed in an HDD to execute self servo writing, an accurate servo signal is obtained with reduced device costs.
In the above-described method, however, it takes extraordinarily long time to write the reference servo signal, producing a problem of high costs due to low productivity. In order to cope with this problem, a method has been proposed in which servo information is recorded altogether on a magnetic recording medium by a magnetic transfer technology using a master disk carrying the servo information instead of writing the servo information using a magnetic head. Japanese Unexamined Patent Application Publication No. 2002-083421, for example, discloses a method of transferring servo information to a perpendicular magnetic recording medium using a master disk carrying the servo information in a servo pattern formed of a ferromagnetic material.
FIG. 13 illustrates a principle of magnetic transfer called an edge transfer process to a perpendicular magnetic recording medium.
Referring to FIG. 13, a master disk 300 for magnetic transfer is provided with a pattern 305 of protrusions and recesses of a ferromagnetic material, preferably a soft magnetic material. A medium 200 to be transferred is made in close contact with the master disk 300 and an external magnetic field 406 is applied by magnets 400 generating a leakage magnetic flux 407 which penetrates into the medium 200. A magnetic recording layer 208 of the medium 200 is magnetized as indicated by the reference numeral 209, transferring a magnetic signal according to the pattern 305 of the master disk 300.
The magnets 400 are arranged above and below the master disk 300 and the medium 200 to be transferred. The upper and lower magnets are rotated together transferring entire information at once, as shown in FIG. 13.
Another method of magnetic transfer called a bit transfer process is also known as shown in FIGS. 14(a) and 14(b). Describing more in detail, a first magnetic field is applied in an approximately perpendicular direction to the surface of the medium 200 to be transferred by a magnet 400 as shown in FIG. 14(a), to magnetize the medium 200 to be transferred in one direction (an initialization step).
Then as shown in FIG. 14(b), a master disk 300 is made in close contact with the medium 200 to be transferred, and a magnetic field 406 in a direction opposite to the first magnetic field (that has been applied in the initialization step) is applied by a magnet 400. In this second magnetic field application, magnetic field hardly passes through recessed parts of the ferromagnetic pattern 305 formed on the master disk 300, leaving the magnetization in the direction of the first magnetic field. At protruding parts of the ferromagnetic pattern 305, a large amount of magnetic flux passes through the medium 200 magnetizing the medium in the direction of this second magnetic field as indicated by the reference numeral 209. As a result, a magnetization pattern is transferred according to the pattern of protrusions and recesses, that is the ferromagnetic pattern 305 formed on the surface of the master disk 300 (a transferring step).
A method of manufacturing a master disk 300 is disclosed in Japanese Patent No. 3999436 and Japanese Patent No. 3343343, for example.
Japanese Patent No. 3999436 discloses a method of manufacturing a master disk for magnetic transfer as follows. A disk having photoresist applied thereon is irradiated with an electron beam while rotating, to draw a pattern on the photoresist corresponding to the information to be transferred. After a development process, a pattern of protrusions and recesses is formed to obtain an original disk. After that, nickel electroforming is conducted on the original disk to produce a metal mould, which is then peeled off to obtain a metal disk of nickel. Then, a soft magnetic film is deposited on the pattern of protrusions and recesses on the metal disk to manufacture a master disk for magnetic transfer.
Japanese Patent No. 3343343 discloses a method of manufacturing a master disk for magnetic transfer as follows. A photoresist is applied on a nonmagnetic substrate. The photoresist is patterned corresponding to the information to be transferred by a lithographic method exposing to an electron beam followed by a development process. Subsequently, a configuration of protrusions and recesses corresponding to the information to be transferred is formed on the nonmagnetic substrate by means of a dry etching method or the like. Then, a ferromagnetic thin film is deposited by a sputtering method or the like. After that, the resist film and the excessive ferromagnetic thin film formed on the resist film are removed by a lift-off method, to manufacture a master disk for magnetic transfer with a structure having ferromagnetic thin films embedded at the recessed parts.
In the method of manufacturing a master disk for magnetic transfer, as described above, a pattern is formed using an electron beam lithography apparatus to draw the pattern while rotating the substrate. This electron beam lithography is generally executed while rotating the substrate, thereby scanning the electron beam along concentric circles, and switching the electron beam irradiation ON/OFF (ON/OFF drawing).
Meanwhile, the reference servo signal in the spiral mode as described previously is recorded while moving the actuator arm, resulting in a shape as shown in FIG. 15. Each dot 2 of the pattern written on the spiral track 100 by the magnetic head has a shape of approximately parallelogram in which the side A is tilted with a certain angle with respect to the circumferential direction of the concentric circles. Here, the word “approximately” shows the following meaning. The data tracks are not straight lines. Hence, the sides extending along the circumferential direction have slight curvature deviating from the parallelogram shape. Also, “a shape of approximately parallelogram” includes slightly deformed shape of parallelogram, such as slightly nonparallel sides. In addition, a shape of parallelogram includes a rectangular shape.
Such a figure is difficult to be drawn by the drawing scheme of scanning the electron beam on a concentric circle while switching the electron beam irradiation ON/OFF. Even in the case of drawing an analogous shape, the scanning pitch must be very fine, producing a problem of increased drawing time.